Variation between rice accessions in photosynthetic induction in flag leaves and underlying mechanisms

Several breeding initiatives have sought to improve flag leaf performance as its health and physiology are closely correlated to rice yield. Previous studies have described natural variation of photosynthesis for flag leaves; however, none has examined their performance under the non-steady-state conditions that prevail in crop fields. Photosynthetic induction is the transient response of photosynthesis to a change from low to high light. Rice flag leaf photosynthesis was measured in both steady- and non-steady-state conditions to characterize natural variation. Between the lowest and highest performing accession, there was a 152% difference for average CO2 assimilation during induction (Ā300), a 77% difference for average intrinsic water use efficiency during induction (iWUEavg), and a 185% difference for the speed of induction (IT50), indicating plentiful variation. No significant correlation was found between steady- and non-steady-state photosynthetic traits. Additionally, measures of neither steady-state nor non-steady-state photosynthesis of flag leaves correlated with the same measures of leaves in the vegetative growth stage, with the exception of iWUEavg. Photosynthetic induction was measured at six [CO2], to determine biochemical and diffusive limitations to photosynthesis in vivo. Photosynthetic induction in rice flag leaves was limited primarily by biochemistry

Assessing heritability of biochemical limitations in photosynthesis can help elucidate new targets for improvement

Understanding heritability has historically been critical to the incorporation of traits of interest into plant breeding programmes for improvement. In our previous study, we assessed the heritability of several photosynthetic traits in indica rice, including biochemical limitations of photosynthesis. Here, we discuss that even without directly evaluating sink limitation, there is still great value in this study for understanding how photosynthetic traits in rice could be incorporated into a breeding programme and which traits could be given priority. Additionally, we assert that limitation by Jmax will likely be more relevant than TPU limitation in the immediate future and could be a valuable factor for selection. However, we agree with Fabre and Dingkuhn (2022, https://doi.org/10.1111/pbr.13000) that sink limitation should be further examined within the context of plant improvement in the future

Evaluating natural variation, heritability, and genetic advance of photosynthetic traits in rice (Oryza sativa)

Despite significant advances to harvest index and interception efficiency, photosynthesis has remained largely unimproved through conventional breeding approaches. However, increasing photosynthetic efficiency is a key method for enhancing crop productivity, yield, and sustainability. In this study, photosynthetic and morphological traits were characterized in indica rice to examine natural variation and the potential for hybridization in the future. Additionally, broad-sense heritability (H2) was calculated for photosynthetic traits, including, for the first time, biochemical limitations to photosynthesis. Heritability was high for CO2 assimilation in saturating light and [CO2] (Amax; H2 =.65), the maximum rate of carboxylation (Vc,max; H2 =.63), the maximum rate of electron transport (Jmax; H2 =.68), and triosephosphate utilization (TPU; H2 =.73). Genetic advances of up to 17.7% were estimated, suggesting that it would be possible to not only select for the improvement of biochemical components of photosynthesis but also achieve significant gains in one generation. Heritability was low for CO2 assimilation at ambient [CO2] in saturating light (Asat; H2 =.22), suggesting that rising [CO2] may increase heritability for photosynthesis in rice

Dynamics of photosynthetic induction and relaxation within the canopy of rice and two wild relatives

Wild rice species are a source of genetic material for improving cultivated rice (Oryza sativa) and a means to understand its evolutionary history. Renewed interest in non-steady-state photosynthesis in crops has taken place due its potential in improving sustainable productivity. Variation was characterized for photosynthetic induction and relaxation at two leaf canopy levels in three rice species. The wild rice accessions had 16%–40% higher rates of leaf CO2 uptake (A) during photosynthetic induction relative to the O. sativa accession. However, O. sativa had an overall higher photosynthetic capacity when compared to accessions of its wild progenitors. Additionally, O. sativa had a faster stomatal closing response, resulting in higher intrinsic water-use efficiency during high-to-low light transitions. Leaf position in the canopy had a significant effect on non-steady-state photosynthesis, but not steady-state photosynthesis. The results show potential to utilize wild material to refine plant models and improve non-steady-state photosynthesis in cultivated rice for increased productivity. © 2021 The Authors. Food and Energy Security published by John Wiley & Sons Ltd